Trees are comprised mostly of water. Any Boy Scout who has ever tried to light a campfire with freshly-cut wood knows that such green wood is far too wet to burn. This is because a tree's cellular structure is designed to allow sap (which is mostly water) to flow throughout the tree. Some wood varieties literally drip with moisture when they are cut.
Wet wood is not very stable, and will most assuredly shrink as it dries out. This is the reason why freshly-cut lumber does not work very well for fine woodworking, which requires great precision and fine tolerances. The lumber used in precision woodworking must be dried considerably before it is stable enough to use. This is why lumber manufacturers either store lumber for long periods to air-dry it or bake it in ovens, called kiln-drying.
The Affect of Air Moisture on Woodworking Pieces
Ideal stock for woodworking should be at a state of equilibrium with the environment in which the finished project will reside. There are a few considerations to keep in mind here:
- Wood with a moisture content that is out of balance with the surrounding air can either take moisture from the air or return moisture to the air. Wood that absorbs additional moisture will swell; wood that expels moisture will shrink. In some climates, certain times of the year are much more humid than other times. The upper Midwest of the U.S. is a perfect example: While summers are quite humid, winter air can be very dry—enough so that people will use humidifiers in the winter to put moisture into the air. This difference in seasonal humidity is why doors and dresser drawers will stick in the summertime but move freely in the winter.
- A piece of furniture which is at a state of equilibrium with its environment when it is manufactured in a humid place like Miami will undergo an eventual "culture shock" if it is moved to a dry location like Phoenix. Eventually, the wood will expel a lot of moisture to the air in an attempt to equalize with its environment. If the woodworker doesn't plan accordingly when building the piece, cracking is a very real possibility.
How Does Wood Expand?
Knowing that wood will naturally attempt to equalize with the humidity in its environment, a woodworker must know how the wood will expand. Movement in a piece of stock caused by shifts in moisture will occur across the grain, as opposed to along the grain. That is to say; a 1 x 6 that is 4 feet long will almost always stay 4 feet long. However, depending on the moisture content of the stock and the air (and the variety of wood used), the width and thickness (to a lesser extent) may vary considerably.
Methods for Dealing With Expansion and Shrinkage
When building a carcass for a cabinet, each of the four sides of the box should have the grain-oriented in the same direction. As such, all four sides should grow relatively equally (mainly if all four come from the same piece of original stock). This, however, can cause issues when drawers are used in the cabinet, making the drawers difficult to open and close. This is why most cabinet carcasses are built using plywood, which isn't affected by humidity nearly as much as dimensional lumber.
When gluing up boards to make a tabletop, not only should the grain of each of the boards be in the same direction and the boards matched so that consecutive boards have similar colors, but the end grains should run in opposite directions. In other words, when one board is laid with the end grain (indicative of cupping) facing up, the next board should be facing down, then the next one up, and so on. This will help "balance out" any cupping that may occur as moisture levels change.
When orienting such a tabletop on a structure such as a desk, it should be laid so that the end-grains of the boards are on the two short sides of the table. To connect the top to the structure, screw the front side of the desk so that no movement can occur, but on the opposite (back) side, screws should be affixed in slots that will allow the boards to widen or narrow. Failure to account for such movement may eventually lead to cracking (shrinkage) or excessive cupping (expansion) on the tabletop.